Vascular dilators are commonly used when inserting a catheter into a blood vessel in order to widen the hole formed by the initial needle or scalpel puncture so that it is better able to receive the larger diameter catheter. The use of the dilator gradually enlarges the hole. In a typical procedure, a hollow needle is percutaneously inserted into the blood vessel and an elongate, slender guide member, such as a cannula or a guidewire is advanced through the hollow needle into the blood vessel. The needle then is removed over the guide member leaving the guide member in place in the blood vessel and extending proximally out of and through the patient's skin. The hole left by the needle in the blood vessel typically is too small to permit the catheter to be passed therethrough. A dilator is provided to widen the hole. Dilators commonly are in the form of a flexible plastic tube having a guide member lumen adapted to be passed over the guide member. The dilator is of uniform wall thickness except for a distal portion which tapers in a distal direction to the circular distal outlet opening at the distal end of the dilator. The wall thickness of the dilator at the distal tip is relatively thin to facilitate its entry into the hole made by the needle. As the dilator is advanced over the guide member through the puncture hole, the tapered distal portion presents a progressively wider diameter to the puncture hole, thus gradually enlarging the hole. The catheter may be mounted on and carried by the dilator so that once the dilator has been inserted to its full diameter into the blood vessel, the catheter then may be advanced over the dilator and through the enlarged puncture hole into the blood vessel. The dilator and guide member then may be withdrawn leaving the percutaneously placed catheter in place in the blood vessel.
Although there is a relatively close fit between the guide member and the circular opening defined by the distal tip of the dilator about the guide member, there necessarily is some clearance and it sometimes occurs that the intima of the blood vessel may become pinched or caught between the edge at the distal tip of the dilator and guide member. The blood vessel thus may become damaged, particularly if the pinching is unnoticed and the insertion of the dilator is continued. Therefore, there is a need for a dilator that reduces the risk of such traumatic injury to the blood vessel. It is among the general objects of the invention to provide an improved dilator that reduces the risk of pinching the blood vessel and reduces trauma to the blood vessel.